U.S. patent application number 11/282223 was filed with the patent office on 2006-05-18 for system and method for cmp having multi-pressure zone loading for improved edge and annular zone material removal control.
Invention is credited to David A. Hansen, Jiro Kajiwara, Gerard S. Moloney, Alejandro Reyes, Huey-Ming Wang.
Application Number | 20060105685 11/282223 |
Document ID | / |
Family ID | 24279378 |
Filed Date | 2006-05-18 |
United States Patent
Application |
20060105685 |
Kind Code |
A1 |
Kajiwara; Jiro ; et
al. |
May 18, 2006 |
System and method for CMP having multi-pressure zone loading for
improved edge and annular zone material removal control
Abstract
In one aspect, the invention provides a method for planarizing a
circular disc-type semiconductor wafer or other substrate. The
method includes the steps of pressing a retaining ring surrounding
the wafer against a polishing pad at a first pressure; pressing a
first peripheral edge portion of the wafer against the polishing
pad with a second pressure; and pressing a second portion of the
wafer interior to the peripheral edge portion against the polishing
pad with a third pressure. The second pressure may be provided
through a mechanical member in contact with the peripheral edge
portion; and the second pressure may be a pneumatic pressure
against a backside surface of the wafer. Desirably, the pneumatic
pressure is exerted through a resilient membrane, or is exerted by
gas pressing directly against at least a portion of the wafer
backside surface. A carrier or subcarrier for a CMP apparatus that
includes: a plate having an outer surface; a first pressure chamber
for exerting a force to urge the plate in a predetermined
direction; a spacer coupled to a peripheral outer edge of the
plate; a membrane coupled to the plate via the spacer and separated
from the plate by a thickness of the spacer; and a second pressure
chamber defined between the membrane and the plate surface for
exerting a second force to urge the membrane in a third
predetermined direction. Substrate, such as a semiconductor wafer,
processed or fabricated according to the invention.
Inventors: |
Kajiwara; Jiro; (Cupertino,
CA) ; Moloney; Gerard S.; (Milpitas, CA) ;
Wang; Huey-Ming; (Fremont, CA) ; Hansen; David
A.; (Palo Alto, CA) ; Reyes; Alejandro; (San
Jose, CA) |
Correspondence
Address: |
DORSEY & WHITNEY LLP
555 CALIFORNIA STREET, SUITE 1000
SUITE 1000
SAN FRANCISCO
CA
94104
US
|
Family ID: |
24279378 |
Appl. No.: |
11/282223 |
Filed: |
November 18, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10401272 |
Mar 27, 2003 |
6966822 |
|
|
11282223 |
Nov 18, 2005 |
|
|
|
09570369 |
May 12, 2000 |
6558232 |
|
|
10401272 |
Mar 27, 2003 |
|
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|
Current U.S.
Class: |
451/41 ;
451/285 |
Current CPC
Class: |
B24B 37/32 20130101;
B24B 49/16 20130101; B24B 37/30 20130101 |
Class at
Publication: |
451/041 ;
451/285 |
International
Class: |
B24B 7/30 20060101
B24B007/30; B24B 29/00 20060101 B24B029/00 |
Claims
1. A substrate subcarrier for polishing a substrate against a
polishing pad in a chemical mechanical polishing (CMP) tool, said
subcarrier comprising: a substrate receiving portion to receive
said substrate; a vacuum line drawing a vacuum for holding said
substrate on said substrate receiving portion; and a reservoir
provided in said subcarrier, which collects any polishing slurry or
debris that may be sucked or pulled into said vacuum line.
2. A substrate subcarrier as in claim 1, wherein a reservoir
comprises a slope and an opening for facilitating drainage of the
slurry or debris out of said subcarrier.
3. A substrate subcarrier as in claim 2, wherein fluid is injected
into said reservoir for cleaning inside of said reservoir.
4. A method for carrying a substrate during the polishing of the
substrate against a polishing pad in a CMP tool, said method
comprising: receiving a substrate at a subcarrier receiving
portion; holding said substrate on said substrate receiving portion
with a vacuum pressure; and collecting any polishing slurry or
debris that may be sucked or pulled into said vacuum line into a
reservoir provided in said subcarrier.
5. A method claim 4, wherein the reservoir comprises a slope and an
opening for facilitating drainage of the slurry or debris out of
said subcarrier.
6. A method as in claim 5, wherein fluid is injected into said
reservoir for cleaning inside of said reservoir.
7. A polishing apparatus for polishing a surface of a substrate,
comprising: a rotatable polishing pad; and a substrate subcarrier
including: a substrate receiving portion to receive the substrate
and to position the substrate against the polishing pad; a flexible
member connected to said subcarrier such that the bottom surface of
said flexible member is capable of contacting said substrate when
in operation; and an annular member mechanically coupling a
peripheral portion of said flexible member to said substrate
subcarrier such that a first force applied to said annular member
during operation results in a first pressure exerted against said
substrate in contact with said peripheral portion of said flexible
member.
8. The polishing apparatus of claim 7, wherein said flexible member
comprises a membrane.
9. The polishing apparatus of claim 8, wherein said annular member
comprises a thickened portion of said membrane.
10. A substrate subcarrier for polishing a substrate against a
polishing pad in a CMP tool, said subcarrier comprising: a
substrate receiving portion to receive said substrate; a flexible
member connected to said subcarrier such that the bottom surface of
said flexible member is capable of contacting said substrate when
in operation; and an annular member mechanically coupling a
peripheral portion of said flexible member to said substrate
subcarrier such that a first force applied to said subcarrier
during operation results in a first pressure exerted against said
substrate in contact with said peripheral portion of said flexible
member; and a second pressing member applying a second pressure to
a central portion of said flexible member, thereby applying a
second pressure to said substrate when in operation.
11. A substrate subcarrier as in claim 10, wherein said flexible
member comprises a membrane and said annular member comprises a
thickened portion of said membrane.
12. The subcarrier of claim 11, wherein said flexible member is a
membrane and said membrane is decoupled from said wafer subcarrier
in at least one location by a chamber, wherein said chamber is
capable of being pressurized to exert a pneumatic pressure on said
substrate during operation in a location contacting said decoupled
portion of said flexible member.
Description
RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 10/401,272 filed Mar. 27, 2003, which is a continuation of
U.S. patent application Ser. No. 09/570,369 filed May 12, 2000, now
U.S. Pat. No. 6,558,232.
FIELD OF INVENTION
[0002] This invention pertains generally to systems, devices, and
methods for polishing and planarizing semiconductor wafers, and
more particularly to systems, devices, and methods utilizing
multiple planarization pressure zones to achieving
high-planarization uniformity across the surface of a semiconductor
wafer.
BACKGROUND OF THE INVENTION
[0003] As feature size decreases, density increases, and the size
of the semiconductor wafer increase, Chemical Mechanical
Planarization (CMP) process requirements become more stringent.
Wafer to wafer process uniformity as well as intra-wafer
planarization uniformity are important issues from the standpoint
of producing semiconductor products at a low cost. As the size of
dies increases a flaw in one small area increasing results in
rejection of a relatively large circuit so that even small flaws
have relatively large economic consequences in the semiconductor
industry.
[0004] Many reasons are known in the art to contribute to
uniformity problems. These include the manner in which wafer
backside pressure is applied to the wafer during planarization,
edge effect non-uniformities arising from the typically different
interaction between the polishing pad at the edge of the wafer as
compared to at the central region, and to non-uniform deposition of
metal and/or oxide layers to might desirably be compensated for by
adjusting the material removal profile during planarization.
Efforts to simultaneously solve these problems have not heretofore
been completely successful.
[0005] With respect to the nature of the wafer backside polishing
pressure, hard backed heads were typically used. In many
conventional machines, an insert is provided between the carrier
(or subcarrier) surface and the wafer or other substrate to be
polished or planarized in an attempt to provide some softness in an
otherwise hard backed system. This insert is frequently referred to
as the wafer insert. These inserts are problematic because they
frequently result in process variation leading to
substrate-to-substrate variation. This variation is not constant or
generally deterministic. One element of the variation is the amount
of water absorbed by the insert during a period of use and over its
lifetime. Some process uniformity improvement may be achieved by
initially soaking the insert in water prior to use. This tends to
make the initial period of use more like the later period of use,
however, unacceptable processes variations are still observed.
These process variations may be controlled to a limited extend by
preconditioning the insert with water as described and by replacing
the insert before its characteristics change beyond acceptable
limits.
[0006] Use of the insert has also required fine control of the
entire surface to which the insert was adhered as any
non-uniformity, imperfection, or deviation from planarity or
parallelism of the subcarrier surface would typically be manifested
as planarization variations across the substrate surface. For
example, in conventional heads, an aluminum or ceramic plate would
be fabricated, then lapped and polished before installation in the
head. Such fabrication increases the costs of the head and of the
machine, particularly if multiple heads are provided.
[0007] As the size of structures (feature size) on the
semiconductor wafer surface have been reduced to smaller and
smaller sizes, now typically about 0.2 microns, the problems
associated with non-uniform planarization have increased. This
problem is sometimes referred to as a Within Wafer Non-Uniformity
(WIWNU) problem.
[0008] When so called hard backed planarization heads, that is
heads that press the backside of the semiconductor wafer with a
hard surface, the front surface of the wafer may not conform to the
surface of the polishing pad and planarization non-uniformities may
typically result. Such hard backed head designs generally utilize a
relatively high polishing pressure (for example, pressure in the
range between about 6 psi and about 8 psi) are used, and such
relatively high pressures effectively deform the wafer to match the
surface conformation of the polishing pad. When such wafer surface
distortion occurs, the high spots are polished at the same time as
the low spots give some degree of global uniformity but actually
producing a bad planarization result. That is too much material
from traces in some areas of the wafer will be removed and too
little material from others. When the amount of material removed is
excessive, those die or chips will not be useable.
[0009] On the other hand, when a soft backed head is used, the
wafer is pressed against the polishing pad but as the membrane or
other soft material does not tend to cause distortion of the wafer,
lower polishing pressures may be employed, and conformity of the
wafer front surface is achieved without distortion so that both
some measure of global polishing uniformity and good planarization
may be achieved. Better planarization uniformity is achieved at
least in part because the polishing rate on similar features from
die to die on the wafer is the same.
[0010] While some attempts have been made to utilize soft backed
CMP heads, they have not been entirely satisfactory. In some head
designs, there have been attempts to use a layer of pressurized air
over the entire surface of the wafer to press the wafer during
planarization. Unfortunately, while such approaches may provides a
soft backed head it does not permit independent adjustment of the
pressure at the edge of the wafer and at more central regions to
solve the wafer edge non-uniformity problems.
[0011] With respect to correction or compensation for edge
polishing effects, attempts have been made to adjust the shape of
the retaining ring and to modify a retaining ring pressure so that
the amount of material removed from the wafer near the retaining
ring was modified. Typically, more material is removed from the
edge of the wafer, that is the wafer edge is over polished. In
order to correct this over polishing, usually, the retaining ring
pressure is adjusted to be somewhat lower than the wafer backside
pressure so that the polishing pad in that area was somewhat
compressed by the retaining ring and less material was removed from
the wafer within a few millimeters of the retaining ring. However,
even these attempts were not entirely satisfactory as the
planarization pressure at the outer peripheral edge of the wafer
was only indirectly adjustable based on the retaining ring
pressure. It was not possible to extend the effective distance of a
retaining ring compensation effect an arbitrary distance into the
wafer edge. Neither was it possible to independently adjust the
retaining ring pressure, edge pressure, or overall backside wafer
pressure to achieve a desired result.
[0012] With respect to the desirability to adjust the material
removal profile to adjust for incoming wafer non-uniform
depositions, few if any attempts to provide such compensation have
been made.
[0013] Therefore, there remains a need for a soft backed CMP head
that provides excellent planarization, controls edge planarization
effects, and permits adjustment the wafer material removal profile
to compensate for non-uniform deposition of the structural layers
on the wafer semiconductor substrate.
SUMMARY
[0014] The invention provides a polishing head and a polishing
apparatus, machine, or tool (CMP tool) for polishing or planarizing
a surface of a substrate or other work piece, such as a
semiconductor wafer. The apparatus includes a rotatable polishing
pad, and a wafer subcarrier which itself includes a wafer or
substrate receiving portion to receive the substrate and to
position the substrate against the polishing pad; and a wafer
pressing member including a having a first pressing member and a
second pressing member, the first pressing member applying a first
loading pressure at an edge portion of the wafer against the
polishing pad, and the second pressing member applying a second
loading pressure a central portion of the wafer against the pad,
wherein the first and second loading pressures are different.
Although this wafer subcarrier and wafer pressing member may be
used separately, in a preferred embodiment of the invention, the
polishing apparatus further includes a retaining ring
circumscribing the wafer subcarrier; and a retaining ring pressing
member applying a third loading pressure at the retaining ring
against the polishing pad. The first, second, and third loading
pressures are independently adjustable.
[0015] In another aspect, the invention provides a method for
planarizing a circular disc-type semiconductor wafer or other
substrate. The method includes the steps of pressing a retaining
ring surrounding the wafer against a polishing pad at a first
pressure; pressing a first peripheral edge portion of the wafer
against the polishing pad with a second pressure; and pressing a
second portion of the wafer interior to the peripheral edge portion
against the polishing pad with a third pressure. In another aspect,
the second pressure may be provided through a mechanical member in
contact with the peripheral edge portion; and the second pressure
is a pneumatic pressure against a backside surface of the wafer.
Desirably, the pneumatic pressure is exerted through a resilient
membrane, or is exerted by gas pressing directly against at least a
portion of the wafer backside surface.
[0016] In another aspect, the invention also provides a a
subcarrier for a CMP apparatus that includes: a plate having an
outer surface; a first pressure chamber for exerting a force to
urge the plate in a predetermined direction; a spacer coupled to a
peripheral outer edge of the plate; a membrane coupled to the plate
via the spacer and separated from the plate by a thickness of the
spacer; and a second pressure chamber defined between the membrane
and the plate surface for exerting a second force to urge the
membrane in a third predetermined direction.
[0017] In yet another aspect, the invention provides a carrier for
a substrate polishing apparatus including: a housing; a retaining
ring flexibly coupled to the housing; a first pressure chamber for
exerting a first force to urge the retaining ring in a first
predetermined direction relative to the housing; a subcarrier plate
having an outer surface and flexibly coupled to the housing; a
second pressure chamber for exerting a second force to urge the
subcarrier plate in a second predetermined direction relative to
the housing; the retaining ring circumscribing a portion of the
subcarrier plate and defining a circular recess; a spacer coupled
to a peripheral outer edge of the subcarrier plate outer surface
within the retaining ring circular recess; a membrane coupled to
the subcarrier plate via the spacer and disposed within the
circular recess, the membrane separated from the subcarrier plate
outer surface by a thickness of the spacer, and a third pressure
chamber defined between the membrane and the outer subcarrier plate
surface for exerting a third force to urge the membrane in a third
predetermined direction relative to the housing.
[0018] The invention further includes a substrate, such as a
semiconductor wafer, processed or fabricated according to the
inventive method.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a diagrammatic illustration showing an exemplary
multi-head CMP polishing or planarization machine.
[0020] FIG. 2 is a diagrammatic illustration showing a conventional
CMP head.
[0021] FIG. 3 is a diagrammatic illustration showing an embodiment
of soft-backed CMP head having a membrane with a sealed pressure
chamber, wherein FIG. 3A is an embodiment utilizing a membrane
backing plate with pressure chamber recess; FIG. 3B is an
embodiment utilizing an annular corner ring; and FIG. 3C is an
embodiment utilizing a thickened peripheral edge portion of the
membrane to transmit a polishing force.
[0022] FIG. 4 is a diagrammatic illustration showing is an
embodiment of a CMP head having a membrane and orifice.
[0023] FIG. 5 is a diagrammatic illustration showing an embodiment
of a CMP head having a membrane with orifice and a grooved backing
plate.
[0024] FIG. 6 is a diagrammatic illustration showing an embodiment
of a CMP head having a membrane and orifice and cushioning air flow
over the surface of the wafer.
[0025] FIG. 7 is a diagrammatic illustration showing embodiments of
a CMP head having dual sealed pressure chambers.
[0026] FIG. 8 is a diagrammatic illustration showing an embodiment
of a CMP head having a membrane sealed chamber and an annular
tubular pressure ring for adding a differential pressure over a
portion of the membrane and wafer.
[0027] FIG. 9 is a diagrammatic illustration showing an embodiment
of a CMP head having a membrane sealed chamber and a plurality of
annular tubular pressure ring for adding a differential pressure
over a plurality of regions of the membrane and wafer.
[0028] FIG. 10 is a diagrammatic illustration showing a preferred
embodiment of the inventive head having a membrane a sealed
pressure chamber.
[0029] FIG. 11 is a diagrammatic illustration showing an embodiment
of the retaining ring suspension member used in the embodiment of
FIG. 10.
[0030] FIG. 12 is a diagrammatic illustration showing an embodiment
of and alternative torque transfer member that may be used in the
embodiment of FIG. 10.
[0031] FIG. 13 is a diagrammatic illustration showing a detail of
the CMP head of FIG. 10 illustrating the attachment of subcarrier
assembly suspension member in the assembled head.
[0032] FIG. 14 is a diagrammatic illustration showing an embodiment
of the subcarrier assembly suspension member.
[0033] FIG. 15 is a diagrammatic illustration showing an embodiment
of the wafer backside membrane.
[0034] FIG. 16 is a diagrammatic illustration showing an
alternative preferred embodiment of the inventive head having a
membrane with an orifice.
[0035] FIG. 17 is a diagrammatic illustration showing an embodiment
of a membrane backing plate that may be used with the embodiment of
FIG. 16.
[0036] FIG. 18 is a diagrammatic illustration showing a perspective
view of the membrane backing plate of FIG. 17.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0037] The inventive structure and method are now described in the
context of specific exemplary embodiments illustrated in the
figures. The inventive structure and method eliminate many of the
problems associated with conventional head designs using polymeric
insert between the backside of the wafer and the surface of the
wafer subcarrier as well as problems associated with pressure
distribution over the surface of the wafer for soft-backed heads.
The different forces or pressures impart different loading of the
front side surface of the wafer against the polishing pad resulting
in a different rate of removal. The pressure applied to a retaining
ring similarly alters the loading force of the retaining ring
contact surface against the retaining ring and influences material
removal at the edge of the wafer. The inventive structure and
method replace the insert with a flexible film or membrane adjacent
the back side surface of the wafer. In one embodiment, this
membrane forms a sealed enclosure, while in a second embodiment,
the membrane has an opening or orifice such that pressure is
applied at least in part directly against the backside wafer
surface. The use of this backside soft surface pressure chamber or
alternatively direct pressure against the wafer backside surface
along with other elements of the inventive head also permit
polishing at a lower pressure thereby achieving greater within
wafer uniformity. The closed chamber embodiment and the open
orifice embodiment are described in greater detail hereinafter.
[0038] The inventive head also provides separate control of the
amount of material removed from the edge of the wafer as compared
to the amount of material removed near the center of the wafer,
thereby allowing control over a edge uniformity. This control is
achieved in part by providing a head having three separate
substantially independent pressure controls: (i) a backside wafer
pressure exerted against the central portion of the wafer, (ii) a
subcarrier pressure exerted against the peripheral edge of the
backside of the wafer, and (iii) a retaining ring pressure exerted
directly against the polishing pad in an annular region
circumscribing the wafer.
[0039] In the structure to be described, the retaining ring is
supported from the housing via a flexible material so that it may
move vertically with little friction and no binding. Some tolerance
between adjacent mechanical components is provided so that the
retaining ring is able to float on the polishing pad surface in a
manner that accommodates minor angular variations during the
polishing or planarization operation. The subcarrier is likewise
suspended from the housing by a flexible material so that it to may
move vertically with little friction and no binding. As with the
retaining ring, small mechanical tolerances are provided between
adjacent mechanical elements so that the subcarrier is able to
float on the polishing pad surface in a manner that accommodates
minor angular variations during the polishing or planarization
operation. The wafer contacts the subcarrier through a firm
connection only approximate the peripheral edge all the wafer. The
central portion of the wafer interior to the annular peripheral
wafer a edge contacts the subcarrier only through a flexible film
or membrane and cushioning volume of a air or other pneumatic or
hydraulic pressure during the polishing or planarization operation.
In addition to the suspension of the retaining ring and subcarrier
from the head housing, the housing itself is attached to or
suspended from other elements of the planarization machine. Usually
this attachment or suspension is provided by a pneumatic,
mechanical, or hydraulic movement means. For example, a pneumatic
cylinder provides the movement, as is known in the art. This
attachment permits the head as a whole to be moved vertically
upward and downward relative to the surface of the polishing pad so
that the wafer may be placed on the subcarrier prior to polishing
and removed for on the subcarrier at the completion of polishing.
Robotic devices are typically used for this purpose.
[0040] In one embodiment of the invention, the head the lifting and
lowering mechanism is provided with a hard physical stop down which
is adjustable compensates for polishing pad wear and for retaining
ring wear. Compensating for pad wear and/or for retaining ring wear
by adjusting the location of the head as a whole relative to the
pad, rather than utilizing any of the vertical range of movement or
stroke of the subcarrier or of the retaining ring relative to the
housing, is preferable as it maintains the retaining ring and
subcarrier at or near the center of its range of movement thereby
minimizing the likelihood of undesired mechanical effects on the
operation of the head and increasing or stabilizing process
uniformity. Such mechanical effects may for example include, an
increase or decrease in the area of sliding surfaces and their
associated friction, changes in the characteristics of the flexible
couplings between the housing and the retaining ring or between the
housing and the subcarrier, as well as other mechanical effects
caused for example by imperfect assembly or alignment. In essence,
by always positioning the head assembly so that critical
operational elements within the head (such as, the retaining ring,
the subcarrier, and the backside membrane) are operated at or near
a predetermined position, any secondary effects that might
influence the process are reduced.
[0041] Providing this measure of control over the head assembly
relative to the polishing pad also permits longer use of the
polishing pad of any particular thickness, and the use of thicker
pads initially anticipating a longer useful lifetime for such
thicker polishing pad. Of course, in some situations pad
reconditioning may be required for such thicker polishing pads
after a predetermined number of wafers have been polished or based
on the then current properties of the polishing pad.
[0042] Typically adjustment of the few millimeters is sufficient to
accommodate for polishing pad and retaining ring wear. For example,
the ability to just in the range from about 1 mm to about 20 mm is
usually sufficient, were typically the ability to just head
position in the range from about 2 mm to about 8 mm is sufficient
adjustment. These adjustments can be made via an adjustment nut or
screw, an adjustment via a pneumatic or hydraulic actuator using a
change of pressure, via a rack and pinion gear assembly, via a
ratchet mechanism, or via other mechanical adjustment means as are
known in the art. alternatively, position encoders may be utilized
to detect a head lower stop position, which when reached is held by
a clamp or other means. While some electronic control might be
utilized to maintain a detected stop position, such electronic
controls are not preferred as they may be susceptible to noise and
jitter in mechanical position which would construct precise
planarization of the semiconductor wafer or other substrate.
[0043] The inventive CMP head structure and planarization
methodology may be utilized with a CMP machine having a single head
or alternatively having a plurality of heads, such as for example
may be provided in conjunction with a carousel assembly.
Furthermore, the inventive head may be utilized in all manner of
CMP machine's including machines utilizing and orbital motion
polishing component, a circular motion polishing component, a
linear or reciprocating motion polishing component, and
combinations of these polishing motions, as well as in or with
other CMP and polishing machines as are known in the art.
[0044] In FIG. 1, there is shown a chemical mechanical polishing or
planarization (CMP) tool 101, that includes a carousel 102 carrying
a plurality of polishing head assemblies 103 comprised of a head
mounting assembly 104 and the substrate (wafer) carrier assembly
106. We use the term "polishing" here to mean either polishing of a
substrate 113 generally including semiconductor wafer 113
substrates, and also to planarization when the substrate is a
semiconductor wafer onto which electronic circuit elements have
been deposited. Semiconductor wafers are typically thin and
somewhat brittle disks having diameters nominally between 100 mm
and 300 mm. Currently 100 mm, 200 mm, and 300 semiconductor wafers
are used in the industry. The inventive design is applicable to
semiconductor wafers and other substrates at least up to 300 mm
diameter as well as to larger diameter substrates, and
advantageously confines any significant wafer surface polishing
nonuniformities to no more than about the so-called exclusion zone
at the radial periphery of the semiconductor disc. Typically this
exclusion zone is from about 1 mm to about 5 mm, more usually about
2 mm to about 3 mm.
[0045] A base 105 provides support for the other components
including a bridge 107 which supports and permits raising and
lowering of the carousel with attached head assemblies. Head
mounting assembly 104 is installed on carousel 102, and each of the
polishing head assemblies 103 are mounted to head mounting assembly
104 for rotation, the carousel is mounted for rotation about a
central carousel axis 108 and each polishing head assembly 103 axis
of rotation 111 is substantially parallel to, but separated from,
the carousel axes of rotation 108. CMP tool or machine 101 also
includes the motor driven platen 109 mounted for rotation about a
platen drive axes 110. Platen 109 holds a polishing pad 135 and is
driven to rotate by a platen motor (not shown). This particular
embodiment of a CMP tool is a multi-head design, meaning that there
are a plurality of polishing heads for each carousel; however,
single head CMP tools are known, and inventive CMP head and method
for polishing may be used with either a multi-head or single-head
type polishing apparatus.
[0046] Furthermore, in this particular CMP design, each of the
plurality of heads are driven by a single head motor which drives a
chain (not shown), which in turn drives each of the polishing heads
103 via a chain and sprocket mechanism; however, the invention may
be used in embodiments in which each head 103 is rotated with a
separate motor and/or by other than chain and sprocket type drives.
The inventive CMP tool also incorporates a rotary union providing a
plurality of different gas/fluid channels to communicate
pressurized fluids such as air, water, vacuum, or the like between
stationary sources external to the head and locations on or within
the head. In one embodiment, five different gas/fluid channels are
provided by the rotary union. In embodiments of the invention in
which the chambered subcarrier is incorporated, additional rotary
union ports are included to provide the required pressurized fluids
to the additional chambers.
[0047] In operation, the polishing platen 109 with adhered
polishing pad 135 rotates, the carousel 102 rotates, and each of
the heads 103 rotates about their own axis. In one embodiment of
the inventive CMP tool, the carousel axis of rotation 108 is
off-set from the platen axis of rotation 110 by about one inch;
however, this is not required or even desired in all situations. In
one embodiment, the speed at which each component rotates is
selected such that each portion on the wafer travels substantially
the same distance at the same average speed as every other point on
a wafer so as to provide for uniform polishing or planarization of
the substrate. As the polishing pad is typically somewhat
compressible, the velocity and manner of the interaction between
the pad and the wafer where the wafer first contacts the pad is a
significant determinant of the amount of material removed from the
edge of the wafer, and of the uniformity of the polished wafer
surface.
[0048] A polishing tool having a plurality of carousel mounted head
assemblies is described in U.S. Pat. No. 4,918,870 entitled
Floating Subcarriers for Wafer Polishing Apparatus; a polishing
tool having a floating head and floating retainer ring is described
in U.S. Pat. No. 5,205,082 Wafer Polisher head Having Floating
Retainer Ring; and a rotary union for use in a polisher head is
described in U.S. Pat. No. 5,443,416 and entitled Rotary Union for
Coupling Fluids in a Wafer Polishing Apparatus; each of which are
hereby incorporated by reference
[0049] In order to establish the differences between the inventive
CMP head and the CMP method associated with use of embodiments of
the head, attention is first directed to the simplified
prototypical head having conventional design of FIG. 2.
[0050] In the embodiment of FIG. 2, mechanical coil springs are
used to illustrate the application of different forces to different
portions of the head. In fact, though springs may in theory be used
to practice the invention, pneumatic pressure in the form of air
pressure or hydraulic pressure may typically be expected to be used
to provide better pressure uniformity over the desired areas. The
use of springs in this illustration is primarily to provide clarity
of description and to avoid obscuring the invention with
unnecessary conventional detail.
[0051] The conventional CMP head 152 of FIG. 2 includes a housing
top portion 204 and a shaft 206 connecting the housing, and indeed
the remainder of the CMP head, to the motor or other source of
rotary movement as is known in the art. Typically housing 204 would
include an annular shaped housing side portion 205 surrounding the
other components in the head to provide a measure of protection
from polishing slurry, to protect the internal elements from
unnecessary exposure and wear, and to serve as a mechanical guide
for other internal elements, such as for example retaining ring
214. In greatly simplified terms, the retaining ring 214 and the
subcarrier 212 may be considered as being suspended from a flat
horizontal housing plate having an upper surface 208 to which shaft
206 is attached and the lower surface 210 from which retaining ring
214 and subcarrier 212 are suspended.
[0052] Subcarrier 212 is connected to the lower surface 210 of
housing 204 via shafts 216 fixedly connected to upper surface 218
of the subcarrier and extending toward a spherical tooling ball 220
captured by a cylindrical bore 222 in lower surface 210. Tooling
ball 220 may move or slide vertically within the bore 222 to
protect relative vertical motion with housing 204. Bore 222 is
desirably slightly oversized to permit tooling ball 220 to move
without binding and to permit some controlled amount of motion so
that when a plurality of tooling ball and bore sets some angular
motion or tilt of the subcarrier relative to the housing 204 and
polishing pad 226 can occur. However, the fit is sufficiently close
so as not to permit any excessive motion or play that would
undermine the precision of the head. Tooling balls 220 provide a
torque transfer connection between housing 204 and subcarrier 212
so that rotational motion from shaft 206 may be communicated
through subcarrier 212 to the wafer 230 being planarized. The
retaining ring tooling balls, though not illustrated in the
drawings so as to avoid undue complexity that might tend to obscure
the invention, may similarly be used to connect to the housing
[0053] One or more springs 232 are disposed between lower housing
surface 210 and an upper surface 234 of retaining ring 214 and acts
to separate the retaining ring 214 from the top housing 204. As
movement of the housing is constrained during the polishing or
planarization operation, the net effect is to press retaining ring
214 downward against the upper surface of polishing pad 226. In
this particular embodiment, the type of spring 232 or the number of
springs 232 may be adjusted to provide the desired retaining ring
force (F.sub.RR) or retaining pressure (P.sub.RR). However, if
pneumatic pressure is used to urge the retaining ring against the
polishing pad 226, pneumatic pressure exerted downward onto
retaining ring would be adjusted to achieve the downward force of
retaining ring 214 against the polishing pad 226.
[0054] In analogous manner, one or more subcarrier springs 238 are
disposed between lower housing surface 210 and an upper surface 218
of subcarrier 212 and acts to separate the subcarrier from the
housing and to urge the subcarrier toward the polishing pad.
Movement of the housing 208 being constrained during the polishing
operation, the net effect is to press subcarrier 212 downward
toward the upper surface of polishing pad 226. Normally, a separate
pneumatic cylinder is used to move and position the head 152
relative to the polishing pad 226. This movement is used for
example, to position (lower) the head close to the polishing pad
after the wafer or other substrate is loaded for planarization, and
to raise the head away from the pad 226 after planarization has
been completed. Advantageously as mechanical stop is provided as a
reference at the lower limit of movement to assure reasonable
repeatability and avoid physical damage to the head or to the
wafers.
[0055] In this conventional configuration, the lower surface of the
subcarrier mounts the semiconductor wafer 230 backside surface 244
either directly, or through an optional polymeric insert 160.
[0056] It will be appreciated that the conventional CMP head of
FIG. 2 provides a retaining pressure (P.sub.RR) of the retaining
ring 214 against the polishing pad 226, and at least theoretically
a single uniform subcarrier pressure (P.sub.SC) between the front
surface of wafer 230 and the surface of the polishing pad. As is
understood by workers having ordinary skill in the art, the wafer
may not actually experience a uniform pressure over its entire
surface due to various factors, including the dynamics associated
with the rotating head and rotating pad, local pad compression,
polishing slurry distribution, and many other factors. It will also
be appreciated by workers having ordinary skill in the art in light
of the description provided here that a uniform planarization
pressure may not yield a uniform planarization result, and that
some controlled planarization pressure variation may be desirable.
Such control however, cannot be achieved with the CMP head or
planarization method of FIG. 2.
[0057] The invention provide several CMP head embodiments and a
novel method of polishing and planarization that is appropriate for
use with the inventive heads and others. Each of the embodiments
provides structure for controllably altering the planarization
pressure over at least two regions of the semiconductor wafer as
well as separately adjusting the downward force of the retaining
ring against the polishing pad. Control of the retaining ring
pressure is known to influence wafer planarization edge
characteristics as it influences the interaction of the wafer and
the polishing pad at the peripheral edge of the wafer. This effect
is indirect as the effect of the retaining ring pressure may only
be extended for a limited distance under the wafer.
[0058] In FIG. 3 are illustrated three related embodiments of the
inventive head, each having a membrane and a sealed pressure
chamber defined between the subcarrier and the membrane. FIG. 3A
illustrates an embodiment with a substantially solid membrane
backing plate 26, and FIG. 3B illustrates an embodiment without a
membrane backing plate 261 where subcarrier force is communicated
from the subcarrier plate 212 to the membrane 250 only at the outer
peripheral surface by an annular corner ring 260. The FIG. 3C
embodiment is similar to the FIG. 3B embodiment except that the
annular corner ring 260 is eliminated and replaced by a thickened
portion 263 of the membrane 250 that transmits the subcarrier
force. It is noted that in some embodiments, the membrane may be
formed of a composite material and or that the corner ring 260 or
other structure may be integrally formed within the edge portion of
the membrane.
[0059] The structure of the embodiment of the inventive CMP head in
FIG. 3A is now described in greater detail. Mechanical coil springs
232, 238 are used to illustrate the application of different forces
to different portions of the head 202. In fact, though springs may
in theory be used to practice the invention, pneumatic pressure in
the form of air pressure, or hydraulic pressure may typically be
expected to provide better planarization results as such pressure
can be uniformly distributed over the desired area and as pressure
may monitored would not tend to change over time or require
frequent maintenance adjustments that mechanical springs would
likely require. The use of springs in this illustration is
primarily to provide clarity of description and to avoid the need
to conventional structure not relevant to the invention.
[0060] The inventive head 202 of FIG. 3 includes a housing top
portion 204 and a shaft 206 connecting the housing and indeed the
remainder of the head to the motor or other source of rotary
movement as are known in the art. Typically housing 204 would
include a side housing portion or skirt 205 surrounding the other
components in the head, to provide a measure of protection from
polishing slurry, to protect the internal elements from unnecessary
exposure and wear, and to serve as a mechanical guide for other
internal elements. Retaining ring 214 and the subcarrier 212 are
generally suspended from a horizontal plate forming the housing
having an upper surface 208 to which shaft 206 is attached and the
lower surface 210 from which retaining ring 214 and subcarrier 212
are suspended.
[0061] Subcarrier 212 is connected to the lower surface 210 of
housing 204 via shafts 216 fixedly connected to upper surface 218
of the subcarrier 212 and extending toward a spherical tooling ball
220 captured by a cylindrical bore 222 in lower surface 210 of
housing top portion 204. Tooling ball 220 may move or slide
vertically within the bore 222 to provide relative vertical motion
(up and down motion relative to the pad) with housing 204. Bore 222
is desirably has a mechanical tolerance to permit tooling ball 220
to move without binding and to permit some controlled amount of
motion so that when a plurality of tooling ball and bore sets (for
example 3 sets) some angular motion or tilt of the subcarrier
relative to the housing 204 and polishing pad 226 can occur.
Tooling balls 220 provide a torque transfer connection between
housing 204 and subcarrier 212 so that rotational motion from shaft
206 may be communicated through subcarrier 212 to the wafer 230
being planarized. The retaining ring, though not illustrated in the
drawings so as to avoid undue complexity that might tend to obscure
the invention, may similarly be connected to the housing using
tooling balls in the same manner as described for the subcarrier.
Other forms of torque or rotational motion coupling structures and
methods are known in the art and may be used.
[0062] One or more springs 232 are disposed between lower housing
surface 210 and an upper surface 234 of retaining ring 214 and acts
to separate the retaining ring from the housing and urge the
retaining ring against pad 226. As movement of the housing is
constrained during the polishing or planarization operation, the
net effect is to press retaining ring 214 downward against the
upper surface of polishing pad 226. In this particular embodiment,
the type of spring 232 and/or the number of springs may be adjusted
to provide the desired retaining ring force (F.sub.RR) or retaining
pressure (P.sub.RR). However, in the preferred embodiment utilizing
pneumatic pressure, pneumatic pressure exerted downward onto the
retaining ring (either directly or indirectly) would be adjusted to
achieve the downward force of retaining ring 214 against the
polishing pad 226.
[0063] In analogous manner, one or more subcarrier springs 238 are
disposed between lower housing surface 210 and an upper surface 218
of subcarrier 212 and acts to separate the subcarrier from the
housing top portion 204. Movement of the housing 208 being
constrained during the polishing operation, the net effect is to
press subcarrier 212 downward toward the upper surface of polishing
pad 226. Unlike retaining ring 214 which has lower surface 240 that
presses directly against polishing pad 226, the subcarrier of the
present invention does not directly contact the polishing pad, and,
in preferred embodiments of the invention does not even directly
contact the backside wafer surface 244 of wafer 230. Rather,
contact is made through a membrane, diaphragm, or other flexible
resilient material at most, and in other embodiments contact is
partially or fully through a layer of pressurized air or gas.
[0064] In the inventive structure, subcarrier 212 functions
primarily to provide a stable platform for the attachment of a
flexible film, diaphragm, or membrane 250. In one embodiment (See
FIG. 3B and FIG. 3C), a chamber 251 is defined between lower
surface 252 of subcarrier 218 and an inner or upper surface 254 of
membrane 250. The opposite or outer surface 256 of membrane 250
contacts the backside surface 244 of semiconductor wafer 230. In
another embodiment (See FIG. 3A), the chamber 251 is defined
between lower surface of membrane backing plate 261 and inner
surface 254 of membrane 250. A source of pressurized air or gas at
force (FBS) or pressure (PBS) and vacuum is coupled to a fitting
267 at the head surface or via a rotary union and coupled to
chamber 251 via a pipe, tube, or other conduit.
[0065] In the alternative embodiment of FIG. 4, the membrane only
partially covers or extends over the backside wafer surface 244 and
an orifice 265 or other opening is provided in the membrane 250. In
this alternative embodiment, no chamber is formed by the structure
of the head itself, rather, backside pressure (P.sub.BS) builds
against the backside wafer surface 244 only when the wafer 230 or
other substrate is loaded onto the head (chucked) for
polishing.
[0066] In another alternative embodiment of FIG. 6, a volume of air
280 or other gas flows to the backside wafer surface of the wafer
is adjusted through the orifice so that air leaks out from between
the membrane 250 and the backside wafer surface such that the wafer
floats on a cushion of air 280.
[0067] Returning to the FIG. 3 embodiment, the inventive structure
permits different portions of outer membrane surface 256 to press
on wafer backside surface 244 with different pressures in the
center portion 281 relative to the edge portion 282 (See FIG. 3A).
In the embodiment of the invention illustrated in FIG. 3B, an
annular or ring shaped edge or corner piece 260 is the disposed at
or near a peripheral edge 262 of the wafer. Although the portion of
membrane 250 extends over corner piece 260 so as to provide a
substantially continuous membrane to wafer contact area, corner
piece 260 is formed from a somewhat firm material so that it
transmits at least some component of the subcarrier force
(F.sub.SC) to or subcarrier pressure (P.sub.SC) to wafer backside
surface 256. Corner piece 260 may, for example, be formed from a
non-compressible or substantially non-compressible material such as
metal, hard polymeric material, or the like; or from a compressible
or resilient material such as soft plastic, rubber, silicone, or
the like materials. Corner piece 260 may alternatively be of the
form of a tubular bladder containing air, gas, fluid, gel, or other
material, and may either have a fixed volume and pressure or be
coupled to a mechanism for altering the volume and/or pressure of
the a air, gas, fluid, gel, or other material so that the firmness,
compressibility, and the like properties may be adjusted to suit
the particular planarization process. The characteristics of the
corner piece 260 by and large determine how much of the subcarrier
force (F.sub.SC) is communicated to the backside surface 244 of
wafer 230. The purpose of this corner piece 260 is to provide means
for adjusting the polishing pressure at the peripheral edge 262 of
wafer 230 separately from the polishing pressure exerted on the
remainder of the wafer so that material removal and edge effects
may be controlled.
[0068] It is noted that even when a substantially noncompressible
material is used for corner piece 260, portions of the membrane 250
in fact may provide some compressibility and resilience that is
beneficial in minimizing any edge transition that might otherwise
occur or at the boundary between the corner piece and the interior
portions of the wafer. The thickness of membrane 250 may be chosen
to provide the desired degree of firmness and resiliency. Different
processes may even benefit from different characteristics. It is
also noted that although the corner piece 260 illustrated in the
embodiment of FIG. 3B is shown as having a rectangular
cross-section, the cross-section may alternatively be tapered or
rounded so as to provide a smooth transition of surface contour and
pressure.
[0069] In the embodiment of FIG. 3A, a membrane backing plate 261
provides the functional characteristic of the annular corner piece
at the peripheral edge 283 of the wafer 230 and also provides
additional support for the wafer when is being held to the head 202
by a vacuum force. The membrane backing plate 261 limits the amount
of bowing that the wafer may be subjected to during the holding or
chucking operation and prevents cracks from forming within the
traces and other structures formed on the wafer front-side surface
245.
[0070] Pneumatic pressure (e.g. air pressure) interposed lower
membrane backing plate surface 261 (See FIG. 3A) or between lower
subcarrier surface 264 (See FIG. 3B and FIG. 3C) and upper membrane
surface 254 provides a downward force onto the backside wafer
surface 244 through membrane 250. In one embodiment of the
invention, the downward backside wafer force (F.sub.BS) is
generated by a pneumatic pressure communicated to cavity 251
through a bore, orifice, tube, conduit, pipe, or other
communication channel 272 via fitting 267 and or a rotary union to
an external source. This backside pressure is uniformly distributed
over the surface of the wafer interior to annular corner piece 260
in the FIG. 3B embodiment, interior to thickened membrane portion
263 in the FIG. 3C embodiment, and is uniformly distributed over
the surface of the wafer in cavity 251 formed between a recess 279
in the lower membrane backing plate 261 and the upper membrane
surface 254 in the FIG. 3A embodiment having the membrane backing
plate.
[0071] It will be appreciated that wafer 230 experiences a pressure
related to the subcarrier pressure (P.sub.SC) near its peripheral
edge 283 as a result of the effective mechanical coupling between
the subcarrier lower surface 252 and an annular shaped portion 285
of membrane 250 stretched over and in contact with the corner ring
piece 260 or with the peripheral edge portions of the membrane
backing plate. It is noted that the membrane backing plate 261 does
not transmit the mechanical force from the subcarrier in its
central interior region owing to the concave recess 279 formed in
its lower surface. Wafer 230 experiences a pressure related to be
backside pressure (PBS) in the center of the wafer and extending
out toward the edge. In the region adjacent the inner radius of the
corner piece 260 or the edge of the concave circular recess in the
membrane backing plate 261, some transition between the two
pressures (P.sub.SC and P.sub.BS) is typically experienced.
[0072] In general, the peripheral wafer edge polishing pressure may
be adjusted to be either greater-than, less-than, or equal-to, the
central backside wafer polishing pressure. In addition, the
retaining ring pressure (P.sub.RR) may also generally be
greater-than, less-than, or equal-to either the central wafer
polishing pressure or the edge peripheral polishing pressure. In
one particular embodiment of the invention, the retaining ring
pressure is generally in the range between about 5 and about 6 psi,
more typically about 5.5 psi, the subcarrier pressure is generally
in the range between about 3 psi and about 4 psi more typically
about 3.5 psi, and the wafer backside pressure is generally in the
range between about 4.5 and 5.5 psi, more typically about 5 psi.
However, these ranges are only exemplary as any of the pressures
may be adjusted to achieve the desired polishing or planarization
effects over the range from about 2 psi and about 8 psi. In some
embodiments of the invention, the physical weight of the mechanical
element, such as the weight of the retaining ring assembly and the
weight of the subcarrier assembly may contribute to the effective
pressure.
[0073] An alternative embodiment of the structure is illustrated in
FIG. 3C. In this alternative embodiment, the corner piece 260 is
eliminated and replaced by a thickened portion of membrane 250
which effectively acts as a corner ring or corner piece. The
material properties of the membrane and the thickness (t) and width
(w) of this thickened portion by and large determine what portion
of the subcarrier force is distributed over what portion of the
wafer backside surface. Again, while a generally rectangular cross
section of the thickened membrane wall is illustrated in the FIG.
3C embodiment, other sectional shapes or profiles of the thickened
portion many advantageously be chosen to provide a desired
magnitude and distribution of subcarrier force. By suitably
selecting the shape, force may be distributed non uniformly, that
is as a function of radial distance, from the peripheral edge to
achieve a desired material removal characteristic. Where justified
by cost or other considerations, even the material properties of
the membrane may be altered as a function of radial distance from
the center (particularly in the region of the thickened wall 263)
to achieve different force transmission properties through the
thickened wall.
[0074] In the embodiment of FIG. 3 (as well as in each other
embodiment described hereinafter) the region of the wafer 230 over
which direct or substantially direct subcarrier force is
communicated to the wafer may be adjusted over a fairly wide range.
For example, the membrane backing plate material and/or the
location of the membrane backing plate recess 279 (FIG. 3A), the
corner portion (FIG. 3B) or thickened membrane wall portion may
generally extend from between about 1 mm and about 30 mm from the
peripheral edge 262, more typically between about 2 mm and about 15
mm, and more usually between about 2 mm and about 10 mm. However in
general, the width or extent of the recess, corner portion, or
thickened membrane wall portion is determined by the desired
results rather than by any absolute limit on physical distance.
These dimensions may desirably be determined empirically during
testing and establishment of wafer process parameters. In one
embodiment of a 200 mm wafer CMP machine, the recess has a diameter
of about 198 mm, while in another embodiment the recess is about
180 mm in diameter. In general, the required dimensions will be
machine and/or process specific and be determined empirically
during development and design of the machine and tuning of the CMP
process.
[0075] Finally, it is noted that although springs where illustrated
as the force generating elements or means for generating the
retaining ring force (F.sub.RR), and subcarrier force (F.sub.SC),
it should be understood that typically springs would not be used
for many reasons. For example, providing matching spring
characteristics for a large number of springs may be problematic in
practical terms, particularly when replacements are required months
or years after the original manufacture. Also, the structure of the
springs will necessarily physically couple the housing, retaining
ring, and subcarrier so that independence of movement may be
compromised. Rather, air or fluid tight chambers or pneumatic or
hydraulic cylinders are provided so that a pneumatic or hydraulic
force or pressure is developed that drives the retaining ring,
subcarrier, and membrane. The manner in which pressure chambers are
utilized and physical coupling between members is reduced are
addressed in the description of the preferred embodiments of the
invention in FIG. 10 and FIG. 16 and other figures related to these
embodiments.
[0076] Several other alternative embodiments that provide separate
retaining ring polishing force, wafer edge polishing force, and
wafer center polishing force are now described. As the general
structure of the embodiments of the invention illustrated in FIG. 4
through FIG. 9 are similar to that of the FIG. 3 embodiment, only
the major differences are described here.
[0077] In the embodiment of FIG. 4, the membrane 250 includes at
least one opening orifice 265 and no closed chamber is defined by
the structure of the head itself. Rather, wafer backside pressure
only builds to urge the wafer against the polishing pad after the
wafer has been chucked (mounted) to the head and pneumatic pressure
has been introduced through orifice 265 behind the wafer. Although
an embodiment with a membrane backing plate 261 is illustrated, it
is understood that this embodiment may alternatively be practiced
with the corner piece 260 or with the thickened membrane edge
portion 263 already described relative to FIG. 3B and FIG. 3C. When
the membrane baking plate is used, the membrane backing plate
optionally but advantageously includes a reservoir 291 that
collects any polishing slurry or debris that may be sucked or
pulled into the line 272 when vacuum is applied to mount and hold
the wafer. This reservoir 291 prevents any such accumulation from
clogging the line. Further benefit is realized by providing
downward sloping sides 292 for the reservoir, and, optionally a
smaller opening to the reservoir 293 than the largest dimension of
the reservoir. These features permit a relatively large reservoir
capacity, while maintaining maximum wafer backside support, and
facilitates drainage of any liquid or slurry out of the line.
[0078] In the embodiment of FIG. 5, the outward facing surface of
the membrane backing plate 261 has grooves 294 machined or
otherwise formed into the surface to communicate vacuum to
different portions of the wafer and to assist testing or sensing
for proper wafer positioning. Raised portions 295 are retained to
support the wafer and prevent excess bowing. This modification is
desirably made since as a result of the orifice, vacuum mounting
and holding of the wafer might be compromised. In one embodiment, a
combination of radial and circumferential grooves 294 is provided.
A wafer presence sensing hole 296 is optionally provided to
determine if a wafer is properly mounted to the head. If vacuum
pressure can be built behind the wafer, the wafer is properly
mounted; however, if vacuum cannot be built there is either no
wafer present or the wafer is not properly mounted. Details of such
a grooved membrane backing plate are further described relative to
the embodiment of FIG. 16, with details of a particular membrane
backing plate illustrated in FIG. 17 and FIG. 18.
[0079] The embodiment of FIG. 6 also utilizes a membrane 250 having
at least one opening or orifice 265, and in addition to controlling
the pressure to achieve the desired material removal from the wafer
front-side surface, a flow of air or other gas is adjusted to
maintain a layer of air (or gas) between the wafer backside surface
244 and the outer membrane surface 256. In this embodiment, the
wafer rides on a layer of air. Although only a single orifice 265
is illustrated in the drawing, a plurality or multiplicity of such
orifices may be used. The excess air 280 escapes out from between
the wafer and the membrane at the wafer edge. Additional conduits
may be provided at the retaining ring interface is desired to
collect and return the air. Arrows indicated the flow of air over
the backside surface of the wafer and out the peripheral edge of
the wafer.
[0080] The embodiment of FIG. 7 is a variation on the FIG. 3
embodiment and provides a plurality of pressure chambers (in this
illustration two pressure chambers exerting forces F.sub.BS1,
F.sub.BS2 and their corresponding pressures) chambers against the
wafer backside surface 244. In the embodiment of FIG. 7A, the
embodiment of FIG. 3A is modified by providing a second similar
backing plate 261-2 and membrane 250-2 combination interior to the
first membrane 250-1. The two structures are overlaid in the
central portion so that the pressures even over the central portion
of the wafer may be separately controlled, in addition to control
of the edge and retaining ring pressures. Although the central
chamber 251-2 and membrane 250-2 portion are illustrated as having
a backing plate 2612 similar to backing plate 261-1 provided for
the larger outer membrane 250-1, a different backing plate
structure or no backing plate may alternatively be used. For
example, a simple membrane defining a chamber may be used. It is
also to be understood that one or both of the membranes may be very
thin so that the thickness and separation of the membranes 250-1,
250-2 relative to the backside wafer surface 244 is quite small and
maybe somewhat exaggerated in the FIG. 7A illustration to show the
structure. In one embodiment, the combined thickness of the two
membranes may only be from about 0.5 mm to about 2 mm, though
thinner and thicker combinations may be used. In other embodiments,
the membranes from the different pressure chambers are abutted
rather than overlaid and a separating partition or wall separates
the multiple, typically annularly shaped, chambers. In some of
these multiple chamber embodiments, the separator walls between
adjacent annular pressure chambers or zones will be very thin so
that the separator wall is less likely to introduce a pressure
discontinuity at a zone boundary. In other embodiments, the wall
separating the adjacent annular zones may have a thickened
portion.
[0081] A variation of the structure in FIG. 7A is illustrated in
FIG. 7B which shows only portions of the retaining ring and
subcarrier without other portions of the CMP head. It is noted that
in this embodiment, the outer or edge transition chamber receives a
first pressure, and the inner or back side pressure chamber
receives a second pressure. The retaining ring receives a third
pressure. As already described relative to other embodiments of the
invention, either or both of the edge transition chamber or the
backside chamber may include an opening or orifice. When the edge
transition chamber is to include an opening, such opening is
conveniently provided as an annular ring adjacent to the inner back
side chamber, with the understanding that in this particular
embodiment, the inner and outer membranes do not necessarily
overlap, inner membrane having a circular shape and the outer
membrane having an annular shape circumscribing the inner
membrane.
[0082] A different variation of the multiple center pressure or
differential pressure control concept is provided by the embodiment
illustrated in FIG. 8, where an annular shaped substantially
tubular pressure ring or bladder 255 is disposed between portions
of the membrane backing plate 261 or subcarrier 212, typically
within a groove 257 within the subcarrier, and the pressurized tube
or bladder 257 is used to provide additional pressure to certain
areas where it is desirable to remove additional material. A
channel 259 couples pressurized air (F.sub.BS2) or other fluid from
an external source to the tubular bladder 257. When pressurized,
the tube presses against the inner membrane surface 254 to locally
increase the planarization pressure (P.sub.BS1) otherwise present
by virtue of chamber 251.
[0083] The FIG. 9 embodiment extends this concept even further to
provide for a plurality of abutting or substantially abutting
concentric tubular pressure rings or bladders 255 such that a
region may be polished or planarized at a higher or at a lower
pressure than the surrounding regions. While tubular rings or
bladders having a substantially circular cross section are
illustrated, it is understood that in both the FIG. 8 and FIG. 9
embodiments, the shape of the tube may be conveniently chosen to
have the desired pressure or force profile against the membrane and
hence against the wafer 230. Pressurized gas or fluid (F.sub.BS1,
F.sub.BS2, F.sub.BS3, F.sub.BS4, F.sub.BS5) are adjusted to provide
the desired polishing pressure profile across the wafer surface. In
one embodiment, the tube has a generally circular cross section,
while in a preferred embodiment, the tube has a rectangular cross
section and a substantially flat surface of the tube is pressed
against the membrane. In the embodiment of FIG. 9, the annular
tubes may have different radial extents or widths between inner and
outer diameters.
[0084] While each of these several embodiments have been described
separately, it will be clear to those workers having ordinary skill
in the art in light of the description provided here that elements
and features in one embodiment may be combined with elements and
features in other embodiments without departing from the scope of
the invention.
[0085] These embodiments illustrated some of the important features
of the CMP head un-obscured by particular implementation details.
Once the structure in operation of these embodiments are
understood, the structure, planarization methodology, and
advantages of the embodiment in FIG. 10 and FIG. 16 will be more
readily understood and appreciated.
[0086] Recall in the conventional design of FIG. 2, a similar head
design utilizing a conventional polymeric insert 160 interposed
between lower subcarrier surface 264 and wafer backside surface
244. In this structure, the pressure exerted against the backside
surface 244 of wafer 230 is uniform (or at least intended to be
uniform). No structure or mechanism is provided for altering the
pressure at or near the peripheral edge of the wafer relative to
either the pressure exerted against the central portion of the
wafer or the pressure exerted by retaining ring 214 against the
upper surface of polishing pad 226.
[0087] Having described several alternative embodiments of the
inventive structure relative to FIG. 3 through FIG. 9, and compared
those structures and the planarization methods they provide to
conventional structures, such as the structure in FIG. 2, attention
is now directed to a more detailed description of the two preferred
embodiment of the invention, one utilizing a thin membrane and
sealed pressure chamber (FIG. 10) and the second embodiment (FIG.
16) having a membrane with an open orifice, which though similar to
the embodiments described relative to FIG. 3 and FIG. 5
respectively, provide additional features and advantages over those
embodiments. Those workers having ordinary skill in the art in
light of the description provided here will appreciate that the
alternatives described relative to FIG. 5 through FIG. 9 of these
embodiments may also be made relative to the FIG. 10 and FIG. 16
embodiments.
[0088] By providing the relatively stiff ring of rubber at the
outside edge of the wafer and applying the sub-carrier pressure,
the amount of material removal at the edge can be controlled
relative to the amount of material removed in regions interior to
the edge, such as relative to the center of the substrate.
[0089] The sub-carrier pressure presses the rubber ring against the
wafer backside forming a pressure tight seal. Pressing down to the
wafer through the rubber ring at the edge also permits control of
the wafer edge removal rate relative to the wafer interior or
central removal rate so that edge non-uniformity can be controlled
and limited.
[0090] It is noted that in some head designs that provide wafer
backside pressure using a diaphragm, no known conventional CMP head
provides structure that permits application of differential
pressure at the edge versus at interior regions. In the inventive
structure, a higher subcarrier pressure relative to the backside
pressure increases the amount of material removed relative the to
center of the wafer and a lower subcarrier pressure relative to the
backside wafer pressure decreases the amount of material removed
from the edge relative to the center. These two pressure may be
adjusted either to achieve uniform or substantial uniform material
removal, or where earlier fabrication processes have introduced
some non-uniformity, to achieve a material removal profile from
edge to center that compensates for the earlier introduced
non-uniformities.
[0091] In these embodiments of the invention, the subcarrier is
retained primarily to provide a stable element that will
communicate the subcarrier pressure chamber uniformly to the rubber
ring and hence to the region near the edge of the wafer. (Recall
that embodiments of the invention are provide to adjust the
pressure at the edge so that absolute uniform pressure may not be
desired or provided.) Except for modest flatness requirements at
the peripheral edge where downward pressure is applied to the wafer
through the rubber ring, the flatness and smoothness of the
subcarrier surface are immaterial. The subcarrier may therefore be
a lower-precision and less costly part.
[0092] These structures provide a polishing (or planarization)
apparatus, machine, or tool (CMP tool) for polishing a surface of a
substrate or other work piece, such as a semiconductor wafer. The
apparatus includes a rotatable polishing pad, and a wafer
subcarrier which itself includes a wafer or substrate receiving
portion to receive the substrate and to position the substrate
against the polishing pad; and a wafer pressing member including a
having a first pressing member and a second pressing member, the
first pressing member applying a first loading pressure at an edge
portion of the wafer against the polishing pad, and the second
pressing member applying a second loading pressure a central
portion of the wafer against the pad, wherein the first and second
loading pressures are different. Although this wafer subcarrier and
wafer pressing member may be used separately, in a preferred
embodiment of the invention, the polishing apparatus further
includes a retaining ring circumscribing the wafer subcarrier, and
a retaining ring pressing member applying a third loading pressure
at the retaining ring against the polishing pad. The first, second,
and third loading pressures are independently adjustable.
[0093] The inventive head 302 of FIG. 10 includes a housing 304
including an upper housing plate 308, a lower housing skirt 310,
and an internal housing plate 312. Upper hosing plate 308 attaches
via screws or other fasteners 312, 314 to shaft 306 via a shaft
attachment collar 316. While a simple shaft 306 is illustrated, it
is understood that shaft 306 is generally of conventional design
and includes, for example, bearings (not shown) for rotatably
mounting the shaft to the remainder of the polishing machine, one
or more rotary unions 305 for communication gases and/or fluids
from stationary sources of such gasses or fluids off the head to
the head. An example of the type of shaft and rotary union that may
be used with the inventive head structure is illustrated for
example in U.S. Pat. No. 5,443,416 entitled Rotary Union for
Coupling Fluids in a Wafer Polishing Apparatus by Volodarsky et al,
assigned to Mitsubishi Materials Corporation, and hereby
incorporated by reference.
[0094] In the afore described embodiments, upper housing plate 308
provides a stable mechanical platform from which to suspend or
mount the retaining ring assembly 320 and the subcarrier assembly
350. Lower housing skirt 310 provides protection over the outer
peripheral portions of retaining ring assembly 320 such as
preventing the entry of polishing slurry into the interior of the
head, controls or restricts the horizontal movement of the
retaining ring assembly 320, and is operative to clamp an outer
radial edge portion 324 of the flexible retaining ring assembly
mounting ring 323 to the upper housing plate 308.
[0095] Internal housing plate 312 attaches to the lower surface of
upper housing plate 308, and is operative to clamp an inner radial
edge portion 326 of the flexible retaining ring assembly mounting
ring 323 to the upper housing plate 308. Internal housing plate 312
is also operative to clamp an inner radial edge portion 328 of
flexible subcarrier assembly mounting ring 327 to the inner housing
plate 312 and by virtue of its direct connection to upper housing
plate 308, to upper housing plate 308 as well.
[0096] While the FIG. 3 and FIG. 4 embodiments were described
relative to simple one piece generally cylindrical and annular
shaped subcarrier and retaining ring, the present embodiment
provides somewhat more complex assemblies comprising a plurality of
components to perform these functions. Hence reference to retaining
ring assembly rather than to the retaining ring, and reference to
subcarrier assembly rather they and to subcarrier. The structural
and operational principles already described pertain to these
additional embodiments, and, it is understood that the inventive
features described relative to the embodiments illustrated in FIG.
3 through FIG. 9 may be enhanced and elaborated with the particular
implementation details described relative to the embodiments in
FIG. 10 and FIG. 16.
[0097] Retaining ring assembly 320 comprises a retaining ring 321
which contacts polishing pad 226 on a lower ring wear surface 322
in constraints movement of wafer 230 in the horizontal plane of the
pad 226 by defining a wafer pocket 334 along the interior radial
edge 335. Retaining ring assembly 320 also comprises the generally
annular shaped suspension plate 336 having a lower surface 337 and
an upper surface 338. The lower surface 337 attaches to an upper
surface of retaining ring 338 (the surface opposite to wear surface
321) and the suspension plate extends upward from the lower surface
to upper surface 338 where that surface cooperates with the lower
surface 339 of a clamp 340 to moveably attach the retaining ring
suspension plate 322 to the housing 308 via a generally annular
shaped retaining ring suspension coupling element 325.
[0098] In one embodiment of the invention, the retaining ring
pressure is compensated for retaining ring wear. When a
non-rectangular retaining ring wears away, surface area touching
the pad changes with time and wear. As a result, the pressure
established for the process (for example 5 psi) does not have the
intended effect and should desirably be modified to accommodate the
larger surface. A non-rectangular retaining ring shape, such as a
retaining ring shape the provides a beveled outer edge, is
preferable as it improves distribution of polishing slurry to the
wafer and pad beneath the wafer you have this angle, you can have
the slurry getting easy. Therefore, retaining ring pressure may be
independently controlled relative to both subcarrier pressure at
the edge of the wafer and backside pressure in the more central
regions of the wafer. Desirably, the retaining ring wear pressure
compensation is automated and under computer control, based for
example, either on the number of wafers processed, hours of
operation, manual measurements, or sensors that detect the actual
amount of retaining ring wear.
[0099] In one embodiment, the retaining ring suspension element 325
is molded from a flexible rubber-like material (EPDM material) to
include two annular channels 341, 342 on either side of clamp 340.
These two channels appear as curved loops in cross section (See
detail in FIG. 12) and provide relatively frictionless vertical
movement of the retaining ring assembly relative to the housing 304
and subcarrier assembly 350. Furthermore, this type of suspension
element 325 decouples the movement of the retaining ring assembly
320 and of the subcarrier assembly 350 so that the movements are
independent or substantially independent, except for possible
friction generated at their sliding surfaces.
[0100] The suspension of the retaining ring assembly 320 relative
to the housing 304 is achieved at least in part by clamping an
outer radial edge portion 324 between the portion of the upper
housing 308 in the lower housing skirt 310, such as with screws 344
or other fasteners. In similar manner, an inner radial edge portion
326 is clamped between another portion of the upper housing 308 and
the lower housing skirt 310 such as with screws 345 or other
fasteners. The mid portion 343 of the suspension element 325 is
clamped to between the upper surface of retaining ring suspension
plate 336 and clamp 339 using a screws 346 or other fasteners.
Desirably, edges and corners of the housing 304, retaining ring
suspension plate 336, and clamp 339 are rounded to approximate the
nominal curvature of retaining ring suspension element 325 at that
point of contact to reduce stress on the suspension element and to
prevent wear and prolong life of the element. The channels or loops
341, 342 are sized to provide a range of motion vertically (up and
down relative to the polishing pad) for the retaining assembly
320.
[0101] The movement of the retaining ring assembly 320 is
advantageously constrained to a predetermined range of motion that
is sufficient for wafer loading, wafer unloading, and polishing
operations. While there are a variety of interfering mechanical
structures that might be utilized to limit the range of motion, in
the embodiment illustrated in FIG. 10, a notch 348 in retaining
ring suspension plate 336 is provided to make contact with a mating
protrusion 349 extending from the internal housing plate 312 so
that movement of the retaining ring assembly beyond predetermined
limits is prevented. Such over range protection is desirably
provided to protect internal components, particularly the retaining
ring suspension element 325, from damage or premature wear. For
example, if the entire weight of the retaining ring assembly were
to be supported by the retaining ring suspension element 325, the
retaining ring suspension element 325 would likely be damaged or at
least be subject to premature wear.
[0102] An embodiment of the retaining ring suspension element 325
is illustrated in FIG. 11 which illustrates a perspective and
partial half-sectional view of the element showing mid portion 343,
inner and outer loop or channel portions 342, 343, and inner and
outer radial edge portions 324, 326.
[0103] The subcarrier assembly 350 includes a subcarrier support
plate 351, a membrane backing plate 352 attached to the support
plate 351 by screws 353 or other fasteners, membrane 250, and in
one embodiment, a backside pressure chamber 354 defined generally
between a lower or outer surface 355 of membrane backing plate 352
and an miner surface 356 of membrane 350. Other embodiments of the
backside pressure chamber 354 are provided by the invention and are
described in greater detail below.
[0104] Subcarrier assembly 350 also desirably includes a mechanical
stop 358 in the form of a stop screw or stop bolt 358 that is
attached to support plate 351 and interferingly interacts with a
stop surface 359 of internal-housing plate 312 through a hole 359
in internal housing plate 312 to prevent over extension of the
subcarrier assembly from the housing if the head is lifted away
from the polishing pad 226. The stop bolt 358 is chosen to provide
an appropriate range of motion of the subcarrier within the head
during loading, unloading, and polishing, but not such a large
range of motion that internal elements of the head would be damaged
by over extension. For example, as with the retaining ring
assembly, if the entire weight of the subcarrier assembly 350 were
to be supported by the subcarrier assembly suspension element 360,
the subcarrier suspension element 360 would likely be damaged or at
least be subject to premature wear.
[0105] As described relative the embodiments in FIG. 3 and FIG. 4,
tooling balls or equivalent mechanical structures such as keys,
splines, shims, diaphragms, or the like may be used to couple the
housing 208 to the subcarrier assembly 350 and to the retaining
ring assembly 320 for rotational motion.
[0106] In one alternative embodiment, a thin sheet 329 of material
such as metal (for example, thin stainless steel) is used to
communicate torque to the retaining ring assembly and subcarrier
assembly as illustrated in FIG. 12. This structure permits relative
vertical motion between the housing and the attached retaining ring
assembly or subcarrier assembly while also transferring rotational
movement and torque between the coupled members. The design of such
as metal coupling 339 is such that torque is transferred in only
one rotational direction but as the head is rotated in only one
direction, this limitation is not problematic. Other diaphragm type
couplings may alternatively be used to couple the housing to the
retaining ring assembly and/or to the subcarrier assembly. The
inventive features described herein are not limited to any
particular retaining ring or subcarrier suspension system.
[0107] The mechanical structures of the housing, retaining ring
assembly, and subcarrier assembly are designed to reduce the
footprint of the CMP head. For example, a portion of the retaining
ring suspension plate overlays a portion of the subcarrier support
plate. These and other aspects of the mechanical structure
desirably reduce the size of the head and make possible a smaller
CMP machine generally.
[0108] An outer radial portion 361 of subcarrier assembly
suspension element 360 is attached to an upper surface 366 of
subcarrier support plate 351 by a first clamp 367. The clamp 367
may for example include an annular shaped ring 368 overlying the
outer radial portion 361 and secured by screws 369 through holes
364 in the suspension element 360 to the subcarrier support plate
351. An inner radial portion 362 of subcarrier assembly suspension
element 360 is attached to a lower surface 370 by a second clamp
371. The second clamp 371 may for example include an annular shaped
ring 371 overlying the inner radial portion 362 and secured by
screws 372 through holes 364 in the suspension element 360 to the
subcarrier support plate 351.
[0109] A detailed portion of the inventive CMP head is illustrated
in FIG. 13 which shows, among other features, the exemplary
structure of the subcarrier assembly suspension element 360. This
element is also illustrated in FIG. 14 in a perspective and partial
half-sectional view. In particular, it shows element 360 having a
mid-portion 363 in the form of an annular a loop or channel
portion, and outer and inner radial edge portions 361, 362. Annular
channel 363 which in cross-section appears in the form of a curved
loop provides relatively frictionless vertical movement of the
subcarrier assembly relative to the housing 304 and retaining ring
assembly 320. Furthermore, this type of suspension element 360
desirably decouples movement of the retaining ring assembly 320 and
of the subcarrier assembly 350 so that the movements are
independent, again, except for negligible frictional interference
that may occur at sliding surfaces. Suspension element 360 may also
be formed from EPDM also known as EPR which is a general purpose
rubber material with excellent chemical resistence and dynamic
properties. One variant of EPDM has a tensile strength of 800 psi
and a nominal durometer of between 55 and 65.
[0110] An upper surface 380 of membrane backing plate 352 is
attached to a lower surface 381 of subcarrier support plate 351 by
screws 353 or other fasteners. In one embodiment, a lower or outer
surface 382 of the backing plate (the surface facing the membrane
350) includes a recess or cavity 383 such that when the membrane
350 is attached to the membrane backing plate 352, and the membrane
only contacts the backing plate at the outer radial peripheral
portion near the edge of the backing plate. In embodiment of FIG.
10, the separation or cavity 383 between the membrane 350 and the
membrane backing plate defines a chamber into which pneumatic or
air pressure (positive pressure and negative pressure or vacuum)
may be introduced to effect the desired operation of the head.
[0111] In an alternative embodiment to be described relative to
FIG. 16, the membrane includes at least one hole or orifice 265 so
that no enclosure or chamber is defined, rather pressure is applied
to the wafer backside directly. The membrane 350 in the latter
embodiment being used to limit contamination of slurry into the
head and to assist in sealing or partially sealing the wafer to the
head.
[0112] Recall that in the descriptions of the simplified FIG. 3 and
FIG. 4 embodiments, either a corner portion 260 having
predetermined material properties, a membrane backing plate 261
having a recess 279, or a thickened portion 263 of the membrane
itself where used to provide the desired transmission of force from
the subcarrier proximate the peripheral edge. A similar result is
provided by the membrane backing plate 351 alone or in conjunction
with the membrane 250 which is advantageously stretched across the
membrane backing plate 252 (somewhat in the manner of a drum skin
over a cylindrical frame) and attached by utilizing the membrane
backing plate 351 and the lower surface of the subcarrier support
plate as clamping elements.
[0113] In one embodiment, membrane 250 is molded from EPDM or other
rubber-like material; however other materials may be used. For
example, silicon rubber may be used as well but may occasionally
stick to the silicon wafers in some environments. The membrane
material should generally have a durometer of between about 20 and
about 80, more typically between about 30 and about 50, and usually
from about 35 to about 45, with a durometer of 40 giving the best
results in many instances. Durometer is a measure of hardness for
polymeric materials. A lower durometer represents a softer material
than a higher durometer material. The material should be resilient
and have good chemical resistence as well as other physical and
chemical properties consistent with operation in a CMP
planarization environment.
[0114] In one embodiment, membrane 250, 350 is made from about 0%
to about 5% smaller in diameter, more usually between about 2% and
about 3% smaller in diameter, than the desired installed size and
stretched to the full size (100%) during installation, especially
for lower durometer materials. The membrane as manufactured is
therefore smaller than the diameter when installed so that it is
stretched and taught when installed.
[0115] One embodiment of circular membrane 250 is illustrated in
FIG. 15. Membrane 250 has a nominal thickness as fabricated of
between about 0.2 mm and about 2 mm, more usually between about 0.5
mm and about 1.5 mm, and in one particular embodiment a thickness
of about 1 mm. These dimensions are for the central portion of a
constant thickness membrane and do not include thickened portions
at or near its peripheral edge of some embodiments as described
herein above. The membrane fits over either the corner ring or the
outer edge of the membrane backing plate 261, depending upon the
particular implementation.
[0116] The amount of the membrane that actually touches the wafer
backside may vary depending upon the edge exclusion requirements,
the uniformity of the incoming wafers, the polishing non-uniformity
of the CMP process if operated without differential edge pressure,
and other factors. In typical situations, the amount of membrane
that is in contact with the wafer backside will vary between about
0.5 mm and about 20 mm, more typically between about 1 mm and about
10 mm, and usually between about 1 mm and about 5 mm. However,
these ranges arise from the need to correct process non-uniformity
and neither the inventive structure nor method are limited to these
ranges. For example, if there were reason to provide direct
subcarrier pressure to the outer 50 mm region of the wafer, the
inventive structure and method may readily be adapted for that
situation.
[0117] In embodiments of the inventive head that utilize the
annular or ring shaped corner insert to transmit subcarrier
pressure to the edge of the wafer, the membrane may have
substantially uniform wall thickness on the bottom and side wall
portions. However, when the thickened membrane side wall itself is
used as the force transmission means, then the side wall thickness
should be commensurate with the distance over which the subcarrier
force is to be directly applied to the wafer. In simple terms, if
it is desired that the subcarrier force be applied to the outer 3
mm of the wafer then the membrane side wall thickness should be 3
mm. It will also be appreciated that there may not be a precise
one-to-one relationship between the desired area or zone over which
the subcarrier force is to be applied and the thickness of the
membrane side wall. Some transition in the force or pressure
transmission between the adjacent areas may be expected and indeed
may even be desirable in some circumstances to avoid an abrupt
pressure discontinuity. Also, it may sometimes, though not always,
be desirable to provide a membrane side wall thickness somewhat
less or somewhat more than the distance over which the subcarrier
force is to be applied to provide a desired pressure transition
between subcarrier pressure and wafer backside pressure. For
example, in some instances for a nominal 3 mm wafer outer
peripheral zone over which direct subcarrier pressure is to be
applied, the membrane side wall thickness may be in the range of
between about 2 mm and about 4 mm. It will be understood that these
particular numerical values are exemplary only and that the best
dimensions will depend on such factors as membrane material,
planarization pressures, polishing pad characteristics, type of
slurry, and so forth, and will generally be determined empirically
while developing the CMP machine and process.
[0118] In a general sense, and without benefit of theory, when
F.sub.SC>F.sub.BS, the subcarrier pressure (F.sub.SC) overrides
pressure at the edge of the wafer so that the wafer edge sees
subcarrier pressure (F.sub.SC) and the central portion of the wafer
sees the backside pressure (F.sub.BS). When F.sub.SC<F.sub.BS,
the backside membrane pressure (F.sub.BS) may dominate the
subcarrier pressure (F.sub.SC) when it is great enough. However,
typically the CMP head will be operated with F.sub.SC<F.sub.BS
so that removal of material at the peripheral edge of the wafer is
diminished relative to the amount of material removed in the
central portion. The relative pressures, diameters, and material
properties are adjusted to achieve the desired planarization
results.
[0119] Attention is now directed to a description of the pressure
zones, pressure chambers, and pressures applied to different
portions of the system. By way of summary, a retaining ring
pressure is applied to the urge the lower wear surface of the
retaining ring against the polishing pad, sub-carrier pressure
applied at the outer radial peripheral edge of the wafer, and
backside wafer pressure (or vacuum) applied against the central
back side portion of the wafer. One further pressurized line or
chamber is advantageously used for a head flush to flush polishing
slurry and debris that might otherwise migrate into the head away.
One or more additional zone of pressure may optionally be applied
to a central circular region of the wafer backside or to annular
regions intermediate between the central region and the outer
peripheral region of the wafer backside. Embodiments utilizing such
inflatable generally annular tube or ring shaped bladder are
described elsewhere herein as have rotary unions for communicating
the pressurized fluids to these and other areas of the head.
[0120] In the embodiment just described, backside pressure chamber
354 is defined generally between membrane backing plate 352 outer
surface 355 and an inner surface 356 of membrane 350.
[0121] Attention is now directed to an embodiment of the invention
in FIG. 16, having a membrane with orifice analogous to that
already described relative to FIG. 4. A membrane pressure hole or
orifice is provided in the membrane 250 so that backside pressure
is applied directly against the wafer without the membrane
necessarily touching the wafer backside surface except near the
outer peripheral edge of the wafer where direct subcarrier pressure
is to be applied. In this embodiment, any membrane overlying the
central portion of the wafer during polishing is used primarily to
form a pressure/vacuum seal. That is, when the wafer is being held
against the head during wafer loading and unloading operations. The
size of the membrane orifice may vary from a few millimeters to a
diameter that extends nearly to the outer diameter of the
subcarrier plate.
[0122] As described relative to the FIG. 4 embodiment, a reservoir
prevents polishing slurry from being sucked up into the
pressure/vacuum line during wafer loading. Sloping the edges of the
reservoir facilitates drainage of the slurry back out of the head.
Note that it is expected that the amount of slurry that is sucked
into the reservoir is expected to be small so that only occasional
cleaning is required. Such cleaning may be accomplished manually,
or by injecting a stream or pressurized air, water, or a
combination of air and water to clear the line and the
reservoir.
[0123] The presence of the membrane orifice somewhat complicates
the communication of vacuum to the wafer backside as well as
complicating sensing of proper wafer mounting when the sensing is
accomplished by sensing for vacuum pressure build up. When the
recess in the membrane backing plate is thin, pulling a vacuum from
a central pressure line may result in sealing the membrane against
the backing plate centrally but not communicating the vacuum to
other regions of the wafer. The membrane itself does not exert the
pull as it would were there no orifice. On the other hand, this
problem might be remedied by increasing the thickness or the
membrane backing plate recess or by using the corner insert or
thickened membrane edge embodiments; however, this reduces the
support available to the wafer.
[0124] A better solution is provided by an embodiment of the
membrane backing plate illustrated in FIG. 17 and FIG. 18, where
FIG. 18 is a perspective illustration of the plate illustrated in
FIG. 17. The additional support is desirable to prevent flexing,
bowing, or wrapping of the wafer. Although the wafer substrate
itself may not typically permanently deform, crack, or otherwise be
damaged; the metal, oxide, and/or other structures and lines on the
front side of the wafer may crack if subjected to stress. Hence,
sufficient support is desirably provided to the backside,
particularly when the wafer is pulled up against the diaphragm
during loading before polishing and after polishing before removal
of the wafer.
[0125] One or more orifices or holes are provided near the outer
edge of the membrane backing plate. These serve as bolt holes to
attach the membrane backing plate to the subcarrier plate while
clamping the membrane between them. First and second radial
channels extend from a central orifice that is coupled for
communication with an external pressure/vacuum source that provides
the backside pressure during polishing as well as communicating a
vacuum during wafer mounting before and after polishing. First and
second concentric annular channels intersect the radial channels.
The effect is to communicate pressure and vacuum to the wafer and
yet provide a desired support for the wafer.
[0126] The physical structure of the head also facilitates easy
access for removing the membrane 250 from the sub-carrier support
plate from the outside of the head without any need to disassemble
the head as in many conventional head structures. Recall that the
bolt holes in the membrane backing plate secure the membrane to the
subcarrier plate and are accessible from the exterior of the head.
One or a set of holes are used to check vacuum and wafer presence
or positioning, and another set of holes are used to access screws
or other fasteners that attach the membrane to the head. As the
membrane is a wear item, it will occasionally need to be replaced,
so the ability to replace it from the exterior of the head without
requiring disassembly of the head is advantageous.
[0127] All publications, patents, and patent applications mentioned
in this specification are herein incorporated by reference to the
same extent as if each individual publication or patent application
was specifically and individually indicated to be incorporated by
reference.
[0128] The foregoing descriptions of specific embodiments of the
present invention have been presented for purposes of illustration
and description. They are not intended to be exhaustive or to limit
the invention to the precise forms disclosed, and obviously many
modifications and variations are possible in light of the above
teaching. The embodiments were chosen and described in order to
best explain the principles of the invention and its practical
application, to thereby enable others skilled in the art to best
use the invention and various embodiments with various
modifications as are suited to the particular use contemplated. It
is intended that the scope of the invention be defined by the
claims appended hereto and their equivalents.
* * * * *